1. Field of the Invention
The present invention relates to a gas sensor having a ceramic substrate in which a heater for heating the gas sensor is embedded. Further, the present invention relates to a method of heating the gas sensor. The gas sensor measures oxides such as NO, NO2, SO2, CO2, and H2O and inflammable gases such as CO and CnHm in atmospheric air and exhaust gas discharged from vehicles.
2. Description of the Related Art
Various measuring systems and apparatuses for measuring concentration of gas components in a measurement gas have heretofore been proposed.
For example, according to a method of measuring concentration of NOx in a measurement gas such as exhaust gas, Nox-reducing ability of Rh is utilized. A sensor having a Pt electrode and an Rh electrode formed on an oxygen ion-conductive solid electrolyte such as zirconia is used to measure an electromotive force generated between the electrodes.
However, the sensor is likely to be affected by noises, since the electromotive force changes significantly depending on the change of oxygen concentration in the measurement gas (exhaust gas), and the electromotive force does not change significantly depending on the change of NOx concentration. Further, in order to effectively utilize the NOx-reducing ability of Rh, reducing gases such as CO are required. In general, when the amount of fuel is not enough in combustion, a large amount of NOx is produced. Under the condition, the amount of CO produced in the combustion is small in comparison to the amount of NOx. Therefore, it is not possible to measure the concentration of NOx.
Japanese Laid-Open Patent Publication Nos. 63-38154 and 64-39545 disclose a method of measuring the concentration of NOx. In the method, a pair of electrochemical pumping cells and a sensor cell comprising Pt electrodes and oxygen-ion conductive solid electrolytes and another pair of electrochemical pumping cells and a sensor cell comprising Rh electrodes and an oxygen-ion conductive solid electrolyte are used. The concentration of NOx is measured based on the difference between pumping current values.
Further, according to Japanese Laid-Open Patent Publication Nos. 1-277751 and 2-1543, a pair of sensors each comprising an electrochemical pumping cell and a sensor cell are used. One of the sensors measures a limiting pumping current value at a partial pressure of oxygen in which NOx is not reduced. The other sensor measures a limiting pumping current value at a partial pressure of oxygen in which NOx is reduced. The difference between these limiting pumping current values is calculated. Alternatively, one sensor comprising an electrochemical pumping cell and a sensor cell is used. The sensor selectively measures the limiting pumping current at a partial pressure of oxygen in which NOx is not reduced, or measures the limiting pumping current at a partial pressure of oxygen in which NOx is reduced. The difference between these limiting pumping current values is calculated.
In automobile applications, the temperature of the exhaust gas changes significantly. Therefore, a heater is embedded in the gas sensor so that the operation of the gas sensor is not affected by the change in the temperature of the exhaust gas.
When the heater is turned on, the temperature of the heater is controlled by (1) performing a PID control, (2) controlling the electric current so that the voltage applied to the heater is increased, and (3) applying ramp voltage or staircase voltage to the heater.